1. Field of the Invention
The present invention relates to bistable display devices, and in particular relates to the materials thereof and methods for manufacturing the devices and the materials.
2. Description of the Related Art
Polymer dispersed liquid crystal (PDLC), having several well-known advantages such as polarizer-free, large-area coatable, and less precision requirement on cell gap, is very suitable for flexible display application. However, due to needing a continuous electrical field to retain display image, PDLC is hard to apply as an energy-saving device. Moreover, PDLC hardly offer high display quality, such as contrast ratio, since the difficulty to perfectly match the refractive index between the polymer and the liquid crystal.
Recently a filled nematic liquid crystal display device has been disclosed by mixing some low portion of highly dispersed inorganic nanoparticles, like aerosils, into the liquid crystal molecules phase. It is realized aerosils tend to form larger agglomerates via hydrogen bonding, so there are lots pseudo-bridge linkage between silicon nanoparticles. In initial off-state, the system has a milky appearance because of scattering polydomain formation due to their large optical anisotropy; while in the on state the liquid crystal molecules rotated homeotropically and switched to a transparent state under electrical field, the scattering is low due to the small size of the dispersed particles. Since the bonding between aerosils were reoriented and reformed as a homeotropic frame by following the rotation of liquid crystal molecules, the transparent is retained after removing electrical field, a bistable display was obtained consequently. Therefore filled nematic display is a good candidate to apply in some energy-saving display, like e-Books.
In U.S. Pat. No. 5,729,320, a filled liquid crystal display material and a corresponding device are disclosed, wherein the small quantity of aerosils are well mixed in the liquid crystal molecules, and the memory effect via the aerosils frame structure reached to of 90%.
However, it is true that the operation for switching to the clear state is more easy and stable than back to scattering state. The homeotropic frame structure is firm so that the memory effect in clear state can till quite a long time. In the embodiments of U.S. Pat. No. 5,729,320, heat, laser, ultrasonic wave, or exterior stress are some obvious but indeed impracticable means to break the hydrogen bonding, it is preferable to switch by the electrical field.
Dual frequency liquid crystal (DFLC) is known to use to improve response time of TN LCD. It is notable to be able to change dielectric anisotropic characteristics in different frequency fields. Usually DFLCs show positive dielectric anisotropy under lower frequency, and turn to negative dielectric anisotropy under higher frequency, therein critical frequency called crossover frequency existed as the dielectric anisotropy is zero. There are lots of other commercial dual frequency liquid crystal materials such as DF-02xx, DF-05xx, FX-1001, and FX-1002 from Chisso, MLC-2048 from Merck, for example, the crossover frequency of MLC-2048 is about 13 kHz. Since DFLCs can be driven by electric field in both on and off state, it is found in Electronics Letters 1991, Vol. 27, 13, 1195, a dual frequency liquid crystal molecule is replaced in filled nematic as a method of two-frequency addressing scheme.
However, the absolute value of dielectric anisotropies of the DFLCs are relatively low, especially the negative dielectric anisotropy. That means the threshold voltage will be increased and the scattering dispersed state is hard to completely reformed, as a result, the memory effect and the contrast of the display are badly influenced.
It is benefit to use DFLCs in Filled nematic due to their easy driving scheme, Nevertheless, the contrast is also important for future display application.